1. Field of the Invention
The present invention relates to an air conditioning system in which plural air conditioners are connected to one another through the same bus line, and more particularly to an air conditioning system in which data (signal) communication between each of plural outdoor units and each of plural indoor units and/or between plural outdoor units can be easily performed on the same bus line with avoiding interference between signals from the respective air conditioners on the bus line, and operation data can be easily checked between the units.
2. Description of the Related Art
In a general air conditioner, each of an outdoor unit and an indoor unit is provided with a controller, and the operation of the respective outdoor unit and the indoor unit is controlled by mutually communicating data between the outdoor unit and the indoor unit. A serial communication system has been recently utilized as a communication system, and a large amount of data are communicated (received and transmitted) between the outdoor and indoor units through a relatively narrow (for example, two cores) signal cable.
When an air conditioning system comprising plural air conditioners is set up in a building, typically the outdoor units of the respective air conditioners are arranged on the roof top or the like to be adjacent to one another, and a refrigerant pipe is set up from each outdoor unit to a corresponding indoor unit. In such a case, a signal cable of each air conditioner is also individually laid as in the case of the refrigerant pipe. However, in order to facilitate a cable laying work, etc., it has been attempted to commonly use one bus line for the respective air conditioners. That is, the respective outdoor units and the respective indoor units may be connected to one another through one bus line and each air conditioner so that each air conditioner controls the operation of the outdoor and indoor units thereof through the bus line, whereby the cable laying work can be performed by merely stringing only one signal cable from the roof top or the like, into respective rooms.
According to such an air conditioning system as described above, the data communication between the outdoor unit and the indoor unit of each air conditioner is performed at a predetermined time interval on the basis of a polling signal output from the outdoor unit. In this case, in order to prevent interference (collision) between polling signals output from the respective air conditioners on the bus line, a time lag (phase shift) is applied between the output timings of the polling signals output from the respective outdoor units.
In the above-described air conditioning system, the controller of each outdoor unit contains a clock, and the output timing of the polling signal is managed on the basis of this clock. However, when a long time elapses from the mounting of the air conditioning system, the phase shift between the polling signals is gradually reduced due to the difference of precision between the clocks of the respective outdoor units, and finally the signals of the respective air conditioners may be output substantially at the same timing.
In this case, priority is generally given to the signal of an air conditioner which is output most (more) early in order to avoid interference between the signals, so that the communication of the other air conditioners cannot be performed. That is, since the difference in precision between the clocks is generally very small (for example, about 10 microseconds for 10 seconds), a long time is needed for recovery until a sufficient phase shift is established again (for example, when the output interval of the polling signal is set to 10 seconds and a time for one communication is equal to 1 millisecond, 1000 seconds (about 17 minutes) is needed). For this recovery time, the signal of an air conditioner which is output most early is preferentially used at all times, and the data communication of the other air conditioners which output the polling signals later is intercepted.
Furthermore, in such an air conditioning system as described above, each outdoor unit is provided with a recording device such as a non-volatile memory for recording operation data. A maintenance operator can obtain past operating conditions from the data taken at periodic maintenance times and stored in the recording device. In the maintenance work, the operator detaches an outer panel, etc. from each outdoor unit and then connects a maintenance device to a connector of the outdoor unit to check the operation data of each air conditioner on the basis of a display content of a display unit or a print output. However, if large number of outdoor units are mounted on a big office building or a factory, then the outer panels of all the outdoor units must be detached from all the air conditioners to perform the maintenance work, so that the number of steps for the maintenance work and the maintenance time are increased.